Advances in the understanding of HIV-1 replication and disease progression have spurred development of major therapeutic options for HIV/AIDS treatment and prevention, i.e., highly-active antiretroviral therapy (HAART). Yet, this devastating pathogen remains elusive against what many experts consider the gold-standard, a successful vaccine. One approach to better inform vaccine development is to understand the viral and immune system components that cause some HIV-infected individuals not to progress to AIDS. This unique group of HIV-positive individuals, referred to as long-term non-progressors (LTNP), naturally maintain healthy levels of immune cells (i.e., CD4+ T cells) and low levels of active virus. Interestingly, after ~10-20 years of HIV-1 control, some LTNP spontaneously progress to AIDS, as late progressors (LP). The factors that cause late progression are unclear, but they quite plausibly involve a breakdown in some strong homeostatic balance between host immunological control and viral replication. Better elucidation of this impact of viral replicationon disease progression is of great interest and value to our understanding of and prevention of AIDS. This proposal outlines comparative studies between LTNP and LP to determine whether increased viral fitness leads to disease progression. A distinguishing feature of this proposal is the >8-year longitudinal sample collection from well-defined HIV-1 patient cohorts: 25 LP and 25 LTNP. Blood specimens from these subjects have been collected every ~6 months during natural maintenance of high CD4 T cell counts, low viral loads, and asymptomatic HIV-1 infection. This proposal aims to understand the determinants of viral evolution and fitness over the 4-year transition period to rapid disease progression in LP. To accomplish this, next-generation sequencing techniques will be used to understand the factors at play: 1) presence of a superinfecting virus, 2) emergence of CXCR4/T cell-tropic virus, and/or 3) mutational changes at epitopes targeted by the adaptive immune system. A second aim proposes to assess viral fitness changes associated with the transition to AIDS in LP. This aim will identify temporal relationships between fitness and the aforementioned events, (i.e., super- infection, X4 emergence, and loss of viral epitopes) during disease progression. Elucidating viral correlates associated with LTNP, and their LP counterparts, will undoubtedly provide important advances toward the understanding of virus-host interplay during disease progression. To date, these unique HIV-infected patient groups lack complete understanding. The aims in this proposal seek to infer causative associations, and to develop more detailed mechanistic hypotheses for late-stage progression in AIDS. In turn, fundamental understanding of the loss in viral control, among LP, will offer insight to better inform vaccine development and antiretroviral therapy strategies.